As shown in FIG. 3, the geared motor comprises a motor 1, and a reduction gear 3 connected to a motor shaft 2. A machine device 5 is driven by a gear shaft 4 of the reduction gear 3. In order to control the positioning of the machine device 5 to be within a previously determined operating range, a motor driver 6 drives and controls the motor 1 on the basis of the rotational position data of the motor obtained from a motor shaft encoder 7 via a signal processing circuit 8, and on the basis of a starting point signal obtained from a starting point sensor 9 that detects a machine starting point, which serves as an operation reference of the machine device 5.
When the geared motor 1 is used to perform positioning control of the machine device 5, the motor 1 must be driven and controlled by the motor driver 6 at start-up so that a state is established wherein the machine device 5 is positioned at the machine starting point. A starting point signal obtained from the starting point sensor 9 of the machine device 5 and a Z-phase signal obtained from the motor shaft encoder 7 are used to establish a state wherein the machine device 5 is positioned at the machine starting point.
For example, a state wherein the machine device 5 is positioned at the machine starting point is established in the following manner. First, the motor 1 is rotatably driven in one direction until the machine device 5 returns to the starting point. After the starting point sensor 9 has detected that the machine device has arrived at the starting point, the motor 1 is made to rotate slowly and the machine device 5 is driven in the same direction until a Z-phase signal is output from the motor shaft encoder 7. The motor 1 is subsequently made to rotate slowly in the reverse direction, and after the machine device 5 has reached the machine starting point, the motor 1 is made to rotate a fixed amount in the opposite direction at a very low speed, so that a state is established wherein the machine device 5 is positioned at the machine starting point.
When the positioning operating range of the machine device 5 corresponds to one rotation of the gear shaft in such a positioning operation, the motor shaft 2 must be made to rotate at no more than the reduction ratio of the reduction gear 3. For example, when the reduction ratio of the reduction gear 3 is 1:50, the motor shaft 2 must be made to rotate as many as 50 times.
In particular, when the positioning operating range of the machine device 5 corresponds to multiple rotations of the gear shaft, the motor shaft 2 must be made to rotate at a higher rate. For example, when the positioning operating range of the machine device 5 corresponds to two rotations (720°) of the gear shaft, and the reduction ratio of the reduction gear 3 is 1:50, then in the positioning operation of the machine device 5 at start-up, the motor shaft 2 must be made to rotate as many as 50×360°×2=36,000°, or the starting point signal from the starting point sensor 9 may not be obtained.
In a positioning operation for positioning at the starting point a load side device associated with a conventional geared motor, the motor shaft thus must be made to rotate multiple times, which is problematic in regard to requiring more time and being inefficient.